In recent years, portable cordless products such as a CD player, multimedia player, cellular phone, smartphone, notebook personal computer, tablet device, and video camera have been increasingly miniaturized and made portable. Further, from the standpoint of environmental issues such as air pollution and increased carbon dioxide, hybrid vehicles and electric vehicles have been developed and are at the stage of practical use. Such electronics and electric vehicles require an excellent secondary battery having characteristics such as high efficiency, high output, high energy density, and light weight. As a secondary battery having such properties, various secondary batteries have been developed and researched.
A chargeable and dischargeable secondary battery generally has a structure that prevents direct electrical contact between a positive electrode and a negative electrode by separating the positive electrode (cathode) and the negative electrode (anode) with a porous polymer membrane comprising an organic electrolyte solution.
Until now, V2O5, Cr2O5, MnO2, TiS2, and the like are known as a positive electrode active material of this nonaqueous electrolyte secondary battery. In addition, in lithium ion batteries that are currently commercialized, LiCoO2, LiMn2O4, LiNiO2, and the like are used as a 4-V class positive electrode active material.
On the other hand, as a negative electrode, alkali metals including metallic lithium have been studied so much. This is because, in particular, metallic lithium has a very high theoretical energy density (3861 mAh/g by weight capacity density) and a low charge/discharge potential (−3.045 V vs. SHE) and thus is considered to be an ideal negative electrode material.
Then, as an electrolyte solution, for example, a lithium salt dissolved in a nonaqueous organic solvent is used, which salt has good ionic conductivity and negligible electrical conductivity. During charging, lithium ions move from the positive electrode to the negative electrode (lithium). During discharging, the lithium ions move in the reverse direction back to the positive electrode.
However, using lithium metal as a negative electrode has the following problem. Dendritic lithium (lithium dendrite) precipitates on the lithium surface of the negative electrode during charging. The dendritic lithium grows as the charge and discharge is repeated, causing, for example, detachment from the lithium metal to thereby reduce cycle characteristics. In the worst case, the dendritic lithium grows to the extent that it breaks through the separator, causing a short circuit of a battery, which can cause firing of the battery.
Thus, to use lithium metal as a negative electrode, the problem of lithium dendrite needs to be solved.
Thus, various carbonaceous materials, metals such as aluminum, alloys or oxides thereof, and the like that are able to occlude and release lithium have been studied so much.
However, using these negative electrode materials reduces the capacity as a battery while it is effective for inhibiting the growth of a lithium dendrite.
Consequently, the research and development for using metallic lithium as a negative electrode has still been actively conducted, and a number of improvements such as development of an electrolyte solution and study of a battery-constituting method have been made.
For example, Patent Document 1 (JP 05-258741 A) proposes using a separator with a smaller pore size than that of conventional ones so that a crystal that grows from the negative electrode side does not grow at pore portions in order to inhibit the growth of such a dendritic crystal (dendrite).
Also, Patent Document 2 (JP 09-293492 A) proposes using an expanded porous polytetrafluoroethylene (PTFE) membrane as a battery separator with high porosity, mechanical strength, and heat resistance and treating the surface and internal pore surface of the expanded porous PTFE membrane to modify these surfaces to a hydrocarbon or carbon oxide compound and cover them. This is because lithium metal reacts with PTFE. Namely, the separator (PTFE) is in contact with the whole surface of a negative electrode (lithium), and consequently a reaction occurs at the electrode/separator interface, whereby the lithium electrode surface is covered with a reaction product, which adversely affects the electrolysis/precipitation of the lithium. To solve this problem, Patent Document 2 describes that the reaction between lithium and a PTFE substrate can be prevented by treating the surface and internal pore surface of the expanded porous PTFE membrane to modify these surfaces to a hydrocarbon or carbon oxide compound and cover them.
Further, Patent Document 3 (U.S. Pat. No. 5,427,872) discloses a lithium electrode (anode) secondary battery comprising a first porous separator and a second separator, wherein the first separator is adjacent to the anode and formed by an aliphatic hydrocarbon resin that does not react with lithium and lithium ions, and the second separator is located between the first separator and the cathode and comprises thermoplastic polytetrafluoroethylene that reacts with lithium metal. There is described that, in this secondary battery, when the tip of a lithium dendrite grows from the anode surface and penetrates the first separator to touch the second separator, the tip of the dendrite and the thermoplastic polytetrafluoroethylene of the second separator causes an exothermic reaction, and the thermoplastic polytetrafluoroethylene dissolves to form non-porous blocked parts, which prevents the dendrite from further growing.
However, a means of more reliably inhibiting the growth of a dendrite is still demanded.